In general, alkyl (meth)acrylates are prepared by esterification reactions of alkanols with acrylic acid in the presence of an acid catalyst. Since the esterification reaction is an equilibrium reaction, eliminating the water as produced out of the reactor shifts the reaction equilibrium in the direction of the forward reaction, increasing conversion rates for the alkyl (meth)acrylates.
Among acid catalysts typically used for the esterification reaction are inorganic acids such as sulfuric acid, organic acids such as alkane sulfonic acids, ion exchange resins and the like.
When sulfuric acid having highly corrosive property is used as a catalyst, it requires for a reactor to have walls finished with anti-corrosive materials and to be of a batch type, rather than a continuous type, for proceeding with the esterification reaction. Also, the catalyst after the completion of the reaction should be removed through neutralization with alkaline materials, requiring large amounts of water and thus incurring additional costs for disposal of wastewater.
When the ion exchange resin is used as a catalyst, its catalytic activity sharply decreases at a temperature of 90° C. or higher so that it is impossible to raise a reaction temperature above 90° C. Accordingly, the reaction should be carried out at a relatively low temperature, causing a lower rate of a reaction and a decreased conversion rate for the overall reaction. As a result, an increased amount of unreacted reactants are recirculated to the reactor so that process efficiency deteriorates. Also, productivity declines as the catalyst should be periodically replaced.
In contrast, when using the organic acid catalyst that is less corrosive than the inorganic acids such as sulfuric acid, one can choose a reactor or a column of various materials. Also, after the reaction, it is possible to continuously proceed with a purification process without any additional treatment with salts.
However, the organic acid catalyst can be a reason for rising production costs since its price is from several to dozens times higher than that of the inorganic acid catalyst. In addition, when being used as catalysts, organic acids can be mixed with high boiling materials, i.e., byproducts of the esterification reaction to form tar-like materials, which can be deposited on a surface of a wall of a reactor or an heat exchanger such as a distillation tower to hinder heat exchange or can cause the clogging of pipes in a decomposition reactor for high boiling materials.
Also, part of the organic acid catalyst is discharged together with heavies as waste oils. Since such an amount of the catalyst as discharged should be replenished in the esterification reactor, it is necessary to minimize the discharged amount of the catalyst. In addition, when the waste oils are incinerated to recover some energy in a form of steam, incineration of the waste oils comprising the organic acid catalyst can cause environmental problems. Therefore, they should be subjected to a pretreatment such as a treatment with salts before incineration and sulfur oxides should be disposed separately after the incineration.
Therefore, in the process for preparing an alkyl (meth)acrylate, a wide variety of studies have been conducted as to methods of increasing a recovering and reusing rate of the organic acid catalyst, and reducing the amount of the organic acid catalyst as finally discarded, but they leave much to be desired.